The present invention relates to a turbine meter for use in measurement of flow rate of fluids and, more particularly, to a turbine meter which can measure the flow rates of various fluids having relatively high viscosities, with a function of adjusting the regions of allowable measurement error for various fluids having different viscosities, i.e. a function to optimize the measurable regions for respective fluids.
A turbine meter has been known having a tube and an impeller rotatably disposed in the tube, the impeller being adapted to be rotated at a speed proportional to the flow velocity of the fluid.
In this type of turbine meter, there is a relation expressed by the following equation (1), between the flow rate Q of the fluid and the angular velocity .omega. of the impeller. ##EQU1## where, .alpha.: angle of impeller vane to axis of tube,
r: mean radius of vane, PA0 A: area of annular passage defined by inner and outer peripheries of vane, PA0 Tf: resistance torque of impeller caused by viscosity of fluid against rotation by fluid, PA0 Tm: resistance torque of impeller caused by mechanical resistance against rotation by fluid, PA0 .rho.: density of fluid
In this equation, the term of resistance torque Tm due to the mechanical resistance is negligibly small as compared with the term of resistance torque Tf due to the viscosity resistance.
Thus, the equation (1) can be rewritten without substantial error, as following equation (1'). ##EQU2##
Representing the viscosity coefficient of the fluid by .mu., the above-mentioned resistance torque Tf is given as follows, depending on the states of flow of the fluid.
______________________________________ .rho..sup.1/2 .mu..sup.1/2 Q.sup.3/2 (in case of laminar flow) (2) (in case of flow intermediate (3) Tf.alpha. .rho.Q.sup.2 - K.mu.Q between laminar and turbulent flow) .rho.Q.sup.2 (in case of turbulent flow) (4) ______________________________________
Therefore, the ratio of angular velocity to flow rate .omega./Q are represented, respectively, by the following equations (5), (6) and (7), depending on the states of flow.
______________________________________ ##STR1## (5) ##STR2## ##STR3## (6) ##STR4## (7) ______________________________________
In these equations, K.sub.1, K.sub.2 and K.sub.3 represent, respectively, different constants.
As will be understood from these equations, in case of a turbulent flow, i.e. in the region represented by the equation (7), the ratio .omega./Q is a constant which is independent of the flow rate Q. This means that the angular velocity .omega. of the impeller is proportional to the flow rate Q. It is therefore possible to measure the flow rate accurately, by multiplying the measured angular velocity .omega. by the proportional constant. On the other hand, in case of the laminar flow and the intermediate state of flow, the ratio .omega./Q cannot be independent of the flow rate Q, so that the flow rate Q is not proportional to the angular velocity .omega.. Thus, it is impossible to accurately measure the flow rate, unless a suitable correction is made.
In case of measurement of the flow rate of a relatively low viscosity, the flow of fluid assumes a state of turbulent flow from a relatively low region of flow rate, so that the constant flow rate is obtained even at the low region of flow rate. This means that an accurate measurement of the flow rate can be achieved over a wide range of flow rate.
In contrast to the above, in case of fluids having relatively high viscosities, the state of laminar flow is maintained until the flow rate is increased to a comparatively high level, causing a change of the ratio .omega./Q. Therefore, in case of fluids having relatively high viscosities, the accurate measurement is achieved only over a limited range of flow rate.
Therefore, the conventional turbine meter, which can measure the flow rate of fluids of relatively less viscous fluids such as gasoline, water and the like considerably accurately, cannot provide satisfactorily accurately the flow rate of relatively viscous fluid such as heavy oil or the like over a wide range of flow rate, particularly at a region of relatively small flow rate, unless a suitable measure is taken.
On the other hand, such a measurement system is conceivable as having means for making a non-linear processing of the signal produced by the turbine meter so as to materially make the ratio .omega./Q constant over a wide range of flow rate including region of relatively small flow rate. Such a measurement system is effective if it is intended for use in measurement of flow rate of only one kind of fluid, but it is disadvantageous in that it requires revision of content of the non-linear processing in accordance with the viscosities of the fluids, when it is used for a plurality of different fluids.